Mods come in two options regulated and unregulated: an unregulated box is the same thing as a mechanical mod with a different form. A regulated box mod has a variable voltage / wattage and will remain constant regardless of battery charge. They also will output the same wattage regardless of the resistance of the coil. They do have drawbacks though of having a minimum resistance they can fire.
A mechanical mod simply delivers power directly from a li-ion battery directly to your atomizer without regulation, safety mechanisms, or even wires. A “box mod” is most often a regulated device using one or two li-ion batteries and a chip to regulate voltage all packaged up inside a, you guessed it, box.
While some box mods deliver roughly the same punch as an eGo-Twist, albeit with a considerably longer battery life, there are others that are real powerhouses. The simple answer to the box mod vs. mech question is that a regulated device will deliver the same output throughout the life of the batteries. Furthermore, the batteries tend to last longer and, in most cases, the box mods can deliver higher voltages. This means a greater flexibility in coil designs, since they don’t have to be created to draw a specific amount of power from a single, unregulated battery.
There are several hundred different box mods out on the market today